The present invention relates to components for microelectronic devices and to methods of making such components.
Many components used in microelectronic assemblies include fine leads which are used for connection to other elements of the device. For example, as taught in commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; 5,489,749; 5,536,909; 5,518,964 and 5,619,017 and PCT Publication WO/94/03036, the disclosures of which are hereby incorporated by reference herein, a microelectronic connection component may include a large number of electrically conductive terminals and leads disposed on a suitable support such as a dielectric sheet or a composite element including both metal and dielectric layers. The leads may include connection sections projecting beyond edges of the support or across apertures in the support. The connection sections may be bonded to contacts on a semiconductor chip to thereby connect the chip contacts to the terminals on the component. Most often, the leads are formed principally from a metal such as copper or a copper-based alloy. As disclosed, for example, in commonly assigned U.S. Pat. No. 5,597,470, it is often desirable to provide a layer of a cover metal over some or all of the surfaces of the principal metal portion of the lead. Depending upon the particular application, the cover metal may provide enhanced properties such as easier bonding of the leads to chip contacts or other structures; enhanced fatigue resistance; or enhanced corrosion resistance.
One common procedure for making leads on a support utilizes a thin conductive layer, typically copper, on a dielectric layer such as a dielectric layer of a rigid circuit panel or a flexible circuit panel, commonly referred to as a xe2x80x9ctapexe2x80x9d. A layer of photoresist is applied over the conductive layer and patterned using conventional photographic processes to provide a series of openings in the form of elongated slots at locations where the leads are to be formed. The slots in the photoresist leave portions of the conductive layer at the bottom of each slot exposed. The principal metal such as copper is then deposited in the slots, typically by electroplating the principal metal onto portions of the conductive layer exposed within each slot. The principal metal deposited within each slot fills the bottom portion of the slot. A layer of the cover metal is deposited onto the top surface of the principal metal deposit, facing away from the support by a further electroplating step. The resist is removed and the part is exposed to an etchant which will attack the conductive layer, thereby removing the conductive layer from regions between the leads. In a variant of this process, a layer of the cover metal is deposited on the conductive layer within each slot before deposition of the principal metal, so as to form a cover metal layer on the bottom surface of each principal metal deposit, facing toward the support. After the etching step used to remove the conductive layer, further cover metal may be deposited onto all of the lead surfaces by a further electroplating step.
Typically, the etchant which is used to remove the conductive layer will not attack the cover layer appreciably but will attack the principal metal. The cover layer on the top surface of the principal metal will protect the principal metal from the etchant to some degree. A cover layer on the bottom surface can also provide some protection. However, the vertically-extending edge surfaces of the principal metal are not covered by the cover metal, and these surfaces are attacked by the etchant. Loss of principal metal results in a lead having an irregular cross-sectional shape and xe2x80x9cundercuttingxe2x80x9d or removal of principal metal from beneath the top cover layer, leaving portions of the top cover layer projecting laterally at edges of the lead. Moreover, the principal metal in the finished lead will have cross-sectional area smaller than the cross-sectional area of the original principal metal deposit. All of these phenomena tend to weaken the lead, and to reduce its electrical performance. Moreover, the projecting portions of the top cover layer can break off of the lead, a phenomenon commonly referred to as xe2x80x9cflakingxe2x80x9d. This can cause short circuits between adjacent leads. These phenomena are subject to some variability depending due to variations in the etching process. These phenomena and variations in these phenomena are more significant in the case of fine leads, with small nominal cross-sectional dimensions.
Thus, there has been a need for a lead-forming process which will alleviate the problem of edge surface undercutting. Other metallic elements are also formed by processes similar to the lead-forming process discussed above. For example, metallic terminals are often formed on supports using a process which is the same as the conventional lead-forming process discussed above, except that the openings in the photoresist layer may be in the form of circular discs, squares, ovals or other desired terminal shapes rather than elongated slots. The openings used to form the terminals may be connected with the elongated slot like openings used to form the leads, so as to form the terminals integral with the leads. Etching of the terminal edge surfaces presents the same problem as discussed above with reference to the leads. Similar problems can occur in formation of still other conductive elements, and hence there has been a similar need for a fabrication procedure which alleviates these problems.
Another procedure which is often used in fabrication of microelectronic components is plasma etching of polymeric materials. For example, a reactive plasma can be used to etch polyimide. A part having a polymeric surface is disposed within a plasma treatment chamber and a plasma is formed at subatmospheric pressure by an electrical discharge between a pair of electrodes disposed within the chamber, or between an electrode and a conductive wall of the chamber. Chemically reactive species formed within the plasma attack the polymeric material. This process can be employed to form holes in a polymeric layer. As disclosed in copending, commonly assigned U.S. patent application Ser. No. 09/020,750, filed Feb. 9, 1998, the disclosure of which is hereby incorporated by reference herein, such a process can also be used to remove polymeric material from beneath a lead on a polymeric support, so as to make the lead detachable from the support. It would be desirable to increase the speed and efficiency of such a process. Moreover, it would be desirable to provide such a process with selectivity, so that the process attacks the polymeric layer preferentially in regions adjacent to metallic features on the polymeric layer.
Yet another procedure used in manufacture of microelectronic components is electrochemical stripping. Polymeric materials such as acrylic polymers found in certain photoresists can be removed from the surface of an underlying structure by exposing the surface to a bath of a liquid stripper which attacks the polymer. It would be desirable to provide for selective removal of the photoresist adjacent to metallic features.
One aspect of the invention provides a method of etching a polymeric article having metallic features at a surface of the polymeric article comprising the steps of exposing the surface having the metallic features to a plasma reactive with the polymer constituting the surface. Preferably, the plasma is maintained by applying electrical energy such as an alternating electric field, typically at radio frequencies (xe2x80x9cRFxe2x80x9d) or other frequencies. The plasma selectively etches the polymer adjacent the metallic features. In a method according to the this aspect of the invention, the plasma may be an oxidizing plasma such as a plasma containing one or more constituents selected from the group consisting of oxygen and halogens, most preferably a plasma containing oxygen and fluorine. The polymer desirably is an organic polymer, and may be a photoresist.
The method may include the step of applying an electrical potential to the metallic features of the article being treated. The potential applied to the metallic features desirably is an alternating potential, such as an RF potential or lower frequency potential. The potential applied to the metallic features may provide some or all of the energy input needed to sustain the plasma. Application of the potential to the metallic features enhances the selectivity of the etching process.
A further aspect of the invention provides a method of electrochemically stripping a polymeric material from a surface of an article having metallic features as well as polymer at such surface. The method according to this aspect of the invention desirably includes the steps of contacting the surface of the article with a liquid etchant reactive with the polymeric material in the article and applying an electrical potential to metallic features of the article at the surface during the contacting step so that the liquid etchant preferentially attacks the polymer adjacent said metallic features. The etchant may be an ionic or partially ionic liquid, such as an alkaline liquid of the type commonly employed in stripping photoresists. For example, the etchant may include organic tertiary amines such as triethanolamine. The polymer may includes polar moieties, and may be an acrylic polymer such as a resist of the type commonly used in electrophoretic deposition processes.
Although the present invention is not limited by any theory of operation, it is believed that the selectivity of the polymer removal in processes according to both of the foregoing aspects of the invention arises at least in part from concentration of electrical charge or electric fields at the juncture of the metal and polymer on the surface. Apparently, the discontinuity between the relatively high electrical conductivity of the metal and the far lower electrical conductivity of the polymer causes concentration of charge or fields at the juncture between the metal and the polymer. Regardless of the theory of operation, however, it has been found that the processes discussed above promote selective etching of the polymer adjacent the metal. In particular, where the metal feature at the surface has an edge surface extending downwardly into the polymer, the polymer is selectively removed in an etch region extending downwardly into the polymer alongside the edge surface of the metal, so as to form gaps alongside the edge surfaces.
A further aspect of the invention provides methods of making metallic features, such as leads or other metallic features of a microelectronic connection component, on a support. A method according to this aspect of the invention desirably includes the steps of providing a resist on a surface of the support so as to leave openings in said resist at locations where metallic features are to be formed, and depositing a principal metal on said support in the openings to form a principal metal deposit with a top surface remote from the support and edge surfaces extending downwardly from the top surface toward the support. The method further includes the step of treating the resist so as to remove resist adjacent the principal metal. For example, the surface of the article having the resist and principal metal may be subjected to selective removal treatments as discussed above.
Selective removal of the resist forms gaps between the edge surfaces and the resist and thereby exposes the edge surfaces of the principal metal deposit but leaves at least some of the resist on regions of the support remote from the principal metal deposit. Following selective removal of the resist, a cover metal is deposited onto the exposed edge surfaces of the principal metal deposit. Preferably, the step of depositing the cover metal is performed so as to deposit cover metal on the top surface of the principal metal, facing away from the support as well as on the edge surfaces. Also, cover metal may be provided on the bottom surface of the principal metal by applying cover metal on the support prior to applying the principal metal. Desirably, the cover metal on the top, bottom and edge surfaces of the support is continuous and completely surrounds the principal metal. The method desirably further includes the steps of removing the resist after the step of depositing the cover metal, typically by a nonselective process such as application of a conventional resist stripper or exposure to a plasma effective to strip the resist.
The support may include a conductive layer and the step of depositing the principal metal is performed so as to deposit the principal metal over the conductive layer. In this case the method may include the step of etching the conductive layer to completely or partially release the principal metal deposit from the surface after the step of removing the resist. The cover metal protects the edge surfaces during the step of etching the cover layer. Thus, the formed metallic feature has the intended shape, without the severe undercutting which has occurred in certain processes employed heretofore.